A. Field of the Invention
The present invention relates to high-performance biodegradable materials made from substituted, lightly crosslinked starch polymers which have been stretched, a method of producing substituted, lightly crosslinked starch polymers, a method of producing sheets, films, fibers, threads or other articles that consist at least in part of substituted, lightly crosslinked starch polymers, and to sheets, films, fibers, threads or other articles consisting essentially of substituted, lightly crosslinked oriented starch polymers. The materials exhibit structural and functional stability during processing, storage and use, yet are susceptible to microbial and environmental degradation upon disposal.
B. Biodegradable Materials
Synthetic polymers were invented about 60 years ago. Since that time there has been enormous progress in extending their range of applications. These various chemicals have steadily been further developed to show more valuable properties. One characteristic of synthetic polymers is their durability which can also be a disadvantage. The persistence of plastics in the environment, the shortage of landfill space, concerns over emissions during incineration, and entrapment and ingestion hazards from these materials have spurred efforts to develop biodegradable plastics. The challenge in replacing conventional plastics with biodegradable materials is to design such materials that exhibit structural and functional stability during storage and use, yet are susceptible to microbial and environmental degradation upon disposal.
For several decades it has been a goal of industry to make plastic materials either biodegradable by microorganisms or environmentally degradable by sunlight, moisture, temperature and the like. It is a continuing goal to make materials as fully degradable or compostable as possible. A compostable material is one that under-goes chemical, physical, thermal and/or biological degradation such that it may be incorporated into and is physically indistinguishable from finished compost (humus) and which ultimately mineralizes (biodegrades to CO.sub.2, water and biomass) in the environment like other known compostable matter such as paper and yard waste. It would be highly desirable to provide a material that is compostable especially in a municipal solid waste composting facility where it may undergo biodegradation in the presence of heat, moisture and microorganisms.
In the search of suitable polymers to replace synthetic polymers, starch is of particular interest since this biopolymer should have no adverse impact on human or environmental health. Furthermore, the introduction of starch as a partial replacement for synthetic polymers will contribute to the preservation of nature resources such as petroleum since starch is a renewable resource.
C. Starch and Starch Related Materials
Starch, a natural polymer (C.sub.6 H.sub.10 O.sub.5).sub.n derived from plant materials, is commonly found in the form of tiny microscopic granules (5-25 microns in diameter) comprised of stratified layers of starch molecules. Starch occurs naturally in the roots, seeds, and stems of numerous types of plants, including corn, wheat, rice, millet, barley, and potatoes and constitutes the main carbohydrate reserve of plants. Starch consists of two polysaccharides, amylose and amylopectin, which can be separated according to their differences in solubility.
Amylose is a straight chain polymer of several hundred glucose units linked by a-1,4-glycosidic linkages. Amylose is mainly linear in structure, with molecular weight ranging from about 30,000 up to 1 million, although upper limits of 200,000 to 300,000 are more common.
Amylopectin is highly branched through carbon 6 and has a molecular weight of over 1 million. It is believed to consist of chains of 20 to 25 glucose units linked through carbons 1 and 4, as in amylose, but with the chains connected to each other through the 1,6 linkage. There is usually three times as much as amylopectin as amylose in natural starch, although a much higher proportion of either occurs in certain plants.
From the standpoint of chemical structure, starch differs from cellulose in two major ways: the glucose rings are linked together through carbons 1 and 4 by a-rather than b-linkages, and considerable chain branching occurs through carbon 6. But, like cellulose, the complete hydrolysis of starch yields D-glucose.
Starch may be modified chemically by a process known as crosslinking to provide modified properties. Bi- or polyfunctional reagents are used to covalently bridge, or crosslink, two starch molecules at various locations along their chains to provide viscosity stability as well as acid, heat and shear tolerance.
Another chemical modification process of starch is substitution. Here, the introduction of substituent groups on starch by treatment with monofunctional reagents which react with the hydroxyl groups on starch produces starch esters, starch ethers and starch carbamates. Substituents can lower gelatinization temperature, increase viscosity, improve colloidal properties, and modify the hydrophilic or hydrophobic character.
Starch is thermally processible when a plasticizer, such as water, is added to lower its melting temperature ("Tm") below the decomposition temperature. Starch must be combined with other materials (homopolymers or copolymers) in order to produce a satisfactory extruded film because extrusion of starch alone produces a brittle, water-sensitive foam. Addition of polyethylene or polypropylene are known to add water stability, elasticity, and toughness to processed starch-filled films. Unfortunately, polyethylene and polypropylene are compounds which have been shown not to be biodegradable. As a result, only the starch portion of the composite film biodegrades while the remaining copolymers remain intact. In U.S. Pat. No. 5,087,650, Willett et al., issued Mar. 10, 1992, for example, olefins such as ethylene and propylene are copolymerized with co-monomers such as methyl acrylate, ethyl acrylate, and hexyl acrylate to produce a graft copolymer which is then combined with starch. U.S. Pat. No. 4,839,450, Fanta et al., issued Jun. 13, 1989, discloses starch-poly(methyl acrylate) graft copolymers prepared from hot water-soluble modified starches which can be processed to produce moisture-shrinkable films. The starch-based films are prepared from modified starch products such as partially depolymerized starches and derivatized starches, such as starch ethers, that are soluble in hot water. Vinyl and acrylic monomers are then attached to the starch by graft polymerization. U.S Pat. No. 5,322,866, Mayer et al., issued Jun. 21, 1994, discloses a method of preparing and extruding biodegradable starch blends into biodegradable products. Unprocessed raw starch is combined with biodegradable copolymers such as polyvinyl alcohol (PVOH), or ethylene vinyl alcohol (EVOH), a nucleating agent, and a plasticizer. Graft copolymers are not biodegradable, and thus the final product is actually only partially biodegradable due to decomposition of the starch component. Also, production of biodegradable starch-based products with different ratios of starch and copolymer are not easily manufactured. Furthermore, these conventional techniques of producing a starch-based film require additional steps that take extra time and add to the cost of the final articles.
As described in U.S. Pat. No. 4,673,438, Wittwer et al., issued Jun. 16, 1987, and U.S. Pat. Nos. 4,133,784, issued Jan. 9, 1979, 4,337,181, issued Jun. 29, 1982, and 4,454,268, issued Jun. 12, 1984, all to Otey et al., many starch-based biodegradable formulations use starch that has first been "destructurized" or "gelatinized". Destructurization or gelatinization is accomplished by heating the raw starch granules in the presence of water under elevated pressure. This treatment produces a disordering of the starch granules and allows the starch to be more effectively blended in conventional processing and production steps. U.S. Pat. No. 5,422,387, Toms, issued Jun. 6, 1995, discloses liquid impervious, biodegradable films. The films comprise a blend of an interpenetrated network of destructurized starch with ethylene/acrylic acid copolymers or ethylene/vinyl alcohol copolymers, and an aliphatic polyester such as polycaprolactone. However, destructurizing or gelatinizing the starch requires an additional pass through an extruder which increases the time and cost required to make starch-based biodegradable articles.
While many attempts have been made to produce biodegradable films from petroleum and cellulose or starch derived materials, none has been completely successful because either they are too costly or decompose too slowly for most applications. Starch remains the most abundant, low-cost, biodegradable polymer available, and its use in plastic film production could greatly reduce the demand for petrochemicals and the harmful impact on the environment caused by discarding nonbiodegradable plastic films.
D. Mechanically Deformed Starch-Based Materials
Polymer films have been prepared using starch as a component where the films are subsequently stretched in the manufacturing process. However, most of these methods do not utilize stretching to increase mechanical strength but to decrease the integrity of these polymers. Many of these processes rely on a physical weakening of the polymer through mechanical stretching to achieve some degree of biodegradability. For example, U.S. Pat. No. 5,336,457, Wu et al., issued Aug. 9, 1994, discloses compostable polymeric sheets of biodegradable polymers. The composite film is formed by co-extrusion of top and bottom water insoluble thermoplastic films having a water soluble polymer between the top and bottom and stretching the composite substantially uniformly across and through the depth of the composite to weaken the strength of the composite and to increase the overall area of the composite.
U.S. Pat. No. 5,362,777, Tomka et al., issued Nov. 8, 1994, discloses a process for improving the mechanical properties of single- or multi-ply sheets with at least one ply, consisting at least in part of thermoplastically processable starch, the sheets are stretched monoaxially or biaxially after they have been produced. While this patent discloses the process of stretching a starch-based material to increase mechanical strength, the thermoplastically processable starch (or destructurized) consists essentially of a mixture of starch and/or a starch derivative which is melted by heat and mechanical energy. The composition contains at least 5 percent based on weight of at least one additive or plasticizer added, which lowers the melting point of the mix. In addition, the Tomka patent discloses a process which is performed within a temperature range of 150.degree. C. to 300.degree. C. U.S. Pat. No. 5,415,827, also to Tomka et al., issued May 16, 1995, discloses mixing the thermoplastically processable starch with nonbiodegradable polyolefins such as polyethylene or polypropylene before being processed into sheets and stretched. Hence, the final product also consists of nondegradable materials.